SpaceX set for third mission to space

AS US firm SpaceX prepares for its third launch to the space station on Friday, NASA says there is nothing "particularly challenging" about a mission that a few months ago was unprecedented.

Liftoff is set for 10:10am local time (0210 Saturday AEDT) in Cape Canaveral, Florida, to launch the unmanned Dragon capsule into orbit carrying some 544kg of supplies to the International Space Station.

This is the second of 12 planned trips in NASA's $US1.6 billion contract with SpaceX, the private company owned by billionaire entrepreneur Elon Musk.

Their successful mission last October was the first ever commercial re-supply operation to the ISS, a milestone for US efforts to reduce costs by privatising the space industry.

SpaceX had earlier completed a near flawless test flight to the ISS.

As NASA's ISS program manager Mike Suffredini described plans for the latest 25-day mission, he emphasised that the process has become routine.

"This is not a particularly challenging event," Suffredini said of the launch and berthing at a televised pre-launch press conference on Thursday.

"This unique vehicle has become a very integral part of how we operate and use the space station," he added.

SpaceX president Gwynne Shotwell, also speaking at the press conference, said the final pieces of cargo and supplies were being loaded for the trip, which was scheduled to take less than 24 hours from launch to berthing.

Among the equipment on board, she added, are two grapple bars that will be transported in an unpressurised compartment of the spacecraft, a first for SpaceX.

Also on board will be equipment for 160 experiments to be conducted by the space station crew, which currently consists of two Americans, three Russians, and a Canadian.

On the return flight, Dragon -- the only spacecraft able to bring cargo back to Earth for now -- will be loaded with just over a tonne of materials, including results of medical research.

The capsule is scheduled for a splashdown landing off the coast of California on March 25.